Subject: Juergen Haible
From: "Saved by Internet Explorer 11"
Date: Thu, 10 Apr 2014 10:22:17 -0700

JH FS-1 Frequency Shifter

Common Myths about Frequency Shifters

"A Pitch Shifter produces harmonic sounds, but
a Frequency Shifter will always produce non harmonic
output"

Not true. True is that a PS
preserves the relationship between the harmonics of the input signal, while
the FS shifts each harmonic by a fixed amount. But if the shift frequency is
a harmonic or subharmonic of a monophonic audio signal to be shifted, the
result can be harmonic too. 1V/Octave tracking is important if you want to
do such "harmonic frequency shifting" on a sampled sound, for
example.

"A Frequency Shifter is only good to produce
strange noises"

Not true. Apart
from the tracking method described above, there is the very interesting
region of very small frequency shifts (a fraction of one Hz). Wonderful lush
Phasing can be produced. Slightly detuning a sound with a FS will result in
constant beat rate for all harmonics which creates a very different chorus
effect than a standard (delay, i.e. pitch shifting based) chorus
circuit.

"A cheap FS is enough to experience the typical range of FS
effects."

Not true. Truth is that you can produce the "strange
noises" with a cheap FS like the one of a certain German low budget Modular
Synthesizer manufacturer. But that's not the interesting stuff, really. And
a simple ring modulator will produce strange noises, too.

"A Frequency Shifter needs expensive and exotic
components."

Not true. Even high priced precision multipliers
like the AD633 won't ensure that the carrier is suppressed completely.
There's too many other parameters that have an influence. OTAH, cheap
HF-type balanced modulators and a compander system built from cheap standard
OTAs will do the job. There is a lot of components, and a lot of patience
for measurements and adjustment required, but nothing really expensive. As
with any other analogue musical circuit, the enclosure and front panel
components are the most expensive parts.

Building a high quality Frequency Shifter from
standard components

I had started with a BFO design like the
Moog / Bode and Electronotes shifters, but I ran into severe problems when I
tried to get very low shift (fractions of one Hz). Now, thru-zero operation
was the reason why I tried the BFO method, but several people convinced me
that the slightest jitter in either of the HF oscillators would result in
severe phase errors near zero Hz Beat frequency.

So I designed a base band Sin / Cos VCO that can go
thru zero. This is achieved by full wave rectifying the frequency control
voltage, and changing the "spinning direction" of the sin / cos oscillator
with the Sign of the same control voltage. It took me some time to figure a
practical circuit out, but in the end it was simply combining an idea from
Electronotes (for a "simple", i.e. not quadrature, VCO), with a tri-shaped
quadrature oscillator from Tietze/Schenk, "Halbleiterschaltungstechnik". The
VCO runs from -20kHz to +20kHz, and it is no problem to set it to something
like 0.1 Hz as well. There are linear (thru-zero) inputs and exponential
(V/Oct) Inputs.

Another circuit detail is the use of one cheap chip,
the MC1496, to get both, low-distortion tri-to-sine-shaping (degenerate
emitter method), and multiplying audio signal and carrier, at the same
time.

Carrier suppression of this system wasn't that bad
(never measured it, but I think it's way below -60dB), but I thought a
compander system would be nice to reduce the noise of the multipliers and to
quiet the carrier completly when there is no input signal. (I don't like the
idea of noise gate thresholds, and there's a hint in the Serge catalog that
they do it in a similar way.) So I took the compander circuit of the Roland
VP-330, and adjusted time constants for an application where input- and output
waveforms aren't similar anymore.

The Hilbert Transform approximation (or "dome filter"
in Moog speak) is a direct copy of the Electronotes 12-pole design.

Schematics and Diagrams

Front panel
construction drawing

Hand drawn on squared
paper. Not nice to look at, but helpful to get an overview over the FS-1's
functions.
Note: The "Direction" input is an
additional means to reverse the direction of the VCO. Schemos not shown - it's
just an EXOR function built with another opamp. In total, the FS-1 has three
ways to go "thru zero": The center position of the "Linear" knobs (and the
related modulations), changing between "+" and "-" in each output section, and
a footswitch connected to the "Direction" input.

Linear
/ Exponential Frequency Control / Direction Control
circuit

The thru zero operation works
with a full wave rectifier for the linear control voltage. "Magnitude" sets
the reference current for the exponential current source, and "Sign" switches
the direction of both integrators in the the oscillator. At very small
frequencies, the "sign" can oscillate between the two states at several kHz.
But as this only happens when the integration time constants are in the range
of several seconds, there is enough "inertia" to filter it completely
out.
Unlike other frequency shifters that
work with the BFO method, the exponential CV input can be used to transpose
the frequency shift by a 1V/Octave factor without offset problems.

Note: The linear and exponential modulation paths
are slightly different in the final version than in this drawing. For
example, External Modulation has a switch to linear or expo control, and not
separate inputs. But you can easily figure this out from the
frontpanel.

VCO
Core

Produces two triangle waves with a phase difference of 90
degrees from subaudio thru the whole audio range.

Simulation
Plot of Quadrature Thru Zero VCO

It
takes some time (at least for me) to understand what modulation from positive
to negative frequencies means for each waveform of a Quadrature Oscillator.
This Diagram may help; it shows what happens when the frequency is switched
from positive to negative frequency (Modulation with a step, at 6ms). It
clearly shows how both output waves simply reverse direction, regardles of
their momentary voltage when the step happens. With continuous modulation
waveforms, there would not be an immediate change of direction, but the
frequency would be decreased to zero, and then increased again, with phase
reversed. Think of a spinning wheel that is slowing down and finally
begins to spin in the opposite direction.

Triangle-to-Sine
Shapers and Modulators

An MC1496 is used for both, 4-quadrant
multiplication, and precision triangle to sine shaping. The Sine waveform is
much better than from an overdriven CA3080. Two identical circuits for
multiplication of normal (cos) and quadrature (sin) signals.
Important:
The SigSin and SigCos signals must be AC coupled. The capacitors for this are
at the outputs of the dome filter. This was a deliberate choice: If you don't
want to build the whole Frequency Shifter, but you're probably looking for a
nice standalone Quadrature Thru Zero VCO, or even just for a stand alone
tri->sine converter, you can use this Modulator circuit almost unchanged:
Just connect the Signal inputs to GND, and one input of the multiplier will be
stuck at its limit, and you get the pure waveshaper function. You might have
to reduce the 270k resistors at the output opamp to get a smaller output
level.

Noise
Reduction

The enemy of every Frequency Shifter is a carrier
bleeding thru, especially when the signal input is low. The common solution
for this is a squelch or noise gate at the output.
I can proudly say that
the carrier bleedthru is way down in my FS anyway, but if you listen
carefully, it's there. So rather than trying to push it from -50dB or -60db
(not measured!) to -70dB or more, and rather that cutting low level signals
brutally off with a noise gate, I decided for a compander system. It's widely
taken from Roland's VP330 Vocoder, and after changing the time constants a
little, it made an excellent job. The FS-1 is completely quiet with no input
signal, and the compressor works very smooth. The envelope detector also
serves as optional modulation source for the amount of frequency shift.

Note the "ModGND" connection at the input of the expander. When you have
the modulators on a separate board that the compander, the two grounds are not
exactly the same. Use shielded cable from modulator to expander and connect
the GND from the modulator board to the "ModGND" point on the compander board.
This gave 10dB improovement on carrier suppression in my unit.

Input
Amplifier

A more or less standard SSM2017 application. A 3p4t
rotary switch is used to make a coarse gain adjustment, sensitive enough at
one end for a Shure SM58 from a distance of a few meters, enough attenuation
for a 20Vpp modular synthesizer signal at the other end. A second, unbalanced
input is useful if you want to include the FS-1 into the feedback loop of an
echo machine (Each reflection with increasing or decreasing pitch).

Dome
Filter

Approximation of the Hilbert transform function inside the
audio band. It does not require trimpots, but it takes some work to select the
components. Procedure is described here.

Simulation
Plot of Dome Filter

The diagram shows the
phase of the two all pass filter chains, and the phase difference between
them. It's almost constant at 90 degrees over the whole audio range.

Second
Expander

For a second output, the expander part of the noise
reduction must be duplicated. I used an LM13700 for the compressor and first
expander, because the two OTAs inside are matched. For the second expander, a
CA3080 and a separate voltage to current converter was needed.
The 20k
resistor is no typo. The 13700 branch has 10k, because it needs the double
control current for two Iabc inputs in parallel. Take care for the "ModGND",
too.

Output
Amplifiers

You can select up/down shift, wet/dry mix, and output
level for two outputs independently. Good for echo stuff and stereo
processing. Stereo phasing at a shift of 0.1 ... 0.5 Hz can be quite
impressive.

Modulation
Generator

Rectangle, "Sine" (no precision here !), or envelope can
be used to modulate the amount of frequency shift in a linear or exponential
way.

Not shown:

Some of the smaller stuff, like footswitch input or LED
driver and external modulation input. I hope the drawings resemble the thing
I've built more or less now. I don't remember which opamps exactly there are
at which place, but the ones that are listed in the schemos should work.
There will probably be some error or the other in the drawings. Don't blame
me - make sure that you have understood everything before you start building,
and feel free to give me feedback if you found an error, if you have
comments, or if you just have sucessfully built this thing !

One final remark: The free evaluation version of PSpice was very helpful to
optimize these circuits.

back to my
homepage

All drawings copyright J. Haible
(C)1996